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Ultrasonic Shear Wave


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Ultrasonic Shear WaveUltrasonic Shear Wave, sometimes referred to as Angled Beam Ultrasonic testing, can be used to inspect pipe, critical welds in pressure vessels and plate weldments, and can be used to inspect cracks for depth, size, length and orientation.

This is a common technique used for non-destructive testing, which provides a sensitive, fast and cost effective method to detect, locate and validate a range of large to small defects/deterioration.

Shear Waves are generally more sensitive to critical weld defects, such as cracks and fusion type defects, which are two dimensional in nature. Those types of defects are usually oriented in the weld and become ideal reflectors for Shear Wave Inspection.

Principle

Shear WaveTwo predominant types of waves, or wave modes, are generated within a material with ultrasonic waves: longitudinal and shear. Longitudinal waves (L-Waves) compress and decompress the material in the direction of motion, much like sound waves in air. Shear waves (S-Waves) vibrate particles at right angles compared to the motion of the ultrasonic wave. The velocity of shear waves through a material is approximately half that of the longitudinal waves. The angle in which the ultrasonic wave enters the material determines whether longitudinal, shear, or both waves are produced.

Ultrasonic beam refraction and mode conversion is comparable to light as it passes from one medium to another. Remember how the straw in the glass of water looks broken if observed from the side? The same phenomenon occurs with ultrasonic waves as they are passed into a UUT.

Angle of Reflection and Refraction: Sound energy at ultrasonic frequencies is highly directional and the sound beams used for flaw detection are well defined. In situations where sound reflects off a boundary, the angle of reflection equals the angle of incidence. A sound beam that hits a surface at perpendicular incidence will reflect straight back. A sound beam that hits a surface at an angle will reflect forward at the same angle.

Sound energy that is transmitted from one material to another bends in accordance with Snell's Law of refraction. Again, a beam that is traveling straight will continue in a straight direction, but a beam that strikes a boundary at an angle will be bent according to the formula:

Sin Ø1   V1
-------- = -----
Sin Ø2   V2

where

Ø1 = incident angle in first material
Ø2= refracted angle in second material
V1 = sound velocity in first material
V2 = sound velocity in second material

This relationship is an important factor in angle beam testing

Angle-Beam Testing

Typical angle beam assemblies make use of mode conversion and Snell's Law to generate a shear wave at a selected angle (most commonly 30, 45, 60, or 70 degrees) in the test piece. As the angle of an incident longitudinal wave with respect to a surface increases, an increasing portion of the sound energy is converted to a shear wave in the second material, and if the angle is high enough, all of the energy in the second material will be in the form of shear waves. There are two advantages to designing common angle beams to take advantage of this mode conversion phenomenon. First, energy transfer is more efficient at the incident angles that generate shear waves in steel and similar materials. Second, minimum flaw size resolution is improved through the use of shear waves, since at a given frequency, the wavelength of a shear wave is approximately 60% the wavelength of a comparable longitudinal wave.